The invention herein described is an improvement in or modification of the invention described in our co-pending U.S. application Ser. No. 08/981,385, entitled ELECTROCHEMICAL CELL, filed on Dec. 18, 1997, the contents of which are incorporated herein by reference.
The invention will herein be described with particular reference to a biosensor adapted to measure the concentration of glucose in blood, but it will be understood not to be limited to that particular use and is applicable to other analytic determinations.
It is known to measure the concentration of a component to be analysed in an aqueous liquid sample by placing the sample into a reaction zone in an electrochemical cell comprising two electrodes having an impedance which renders them suitable for amperometric measurement. The component to be analysed is allowed to react directly or indirectly with a redox reagent whereby to form an oxidisable (or reducible) substance in an amount corresponding to the concentration of the component to be analysed. The quantity of the oxidisable (or reducible) substance present is then estimated electrochemically. Generally this method requires sufficient separation of the electrodes so that electrolysis products at one electrode cannot reach the other electrode and interfere with the processes at the other electrode during the period of measurement.
In our co-pending application we described a novel method for determining the concentration of the reduced (or oxidised) form of a redox species in an electrochemical cell of the kind comprising a working electrode and a counter (or counter/reference) electrode spaced from the working electrode by a predetermined distance. The method involves applying an electric potential difference between the electrodes and selecting the potential of the working electrode such that the rate of electro-oxidation of the reduced form of the species (or of electro-reduction of the oxidised form) is diffusion controlled. The spacing between the working electrode and the counter electrode is selected so that reaction products from the counter electrode arrive at the working electrode. By determining the current as a function of time after application of the potential and prior to achievement of a steady state current and then estimating the magnitude of the steady state current, the method previously described allows the diffusion coefficient and/or the concentration of the reduced (or oxidised) form of the species to be estimated.
Our co-pending application exemplifies this method with reference to use of a xe2x80x9cthin layer electrochemical cellxe2x80x9d employing a GOD/Ferrocyanide system. As herein used, the term xe2x80x9cthin layer electrochemical cellxe2x80x9d refers to a cell having closely spaced electrodes such that reaction product from the counter electrode arrives at the working electrode. In practice, the separation of electrodes in such a cell for measuring glucose in blood will be less than 500 microns, and preferably less than 200 microns.
The chemistry used in the exemplified electrochemical cell is as follows:
glucose+GODxe2x86x92gluconic acid+GOD*xe2x80x83xe2x80x83reaction 1
GOD*+2ferricyanidexe2x86x92GOD+2ferrocyanidexe2x80x83xe2x80x83reaction 2
where GOD is the enzyme glucose oxidase, and GOD* is the xe2x80x98activatedxe2x80x99 enzyme. Ferricyanide ([Fe(CN)6]3xe2x88x92) is the xe2x80x98mediatorxe2x80x99 which returns the GOD* to its catalytic state. GOD, an enzyme catalyst, is not consumed during the reaction so long as excess mediator is present. Ferrocyanide ([Fe(CN)6]4xe2x88x92) is the product of the total reaction. Ideally there is initially no ferrocyanide, although in practice there is often a small quantity. After reaction is complete the concentration of ferrocyanide (measured electrochemically) indicates the initial concentration of glucose. The total reaction is the sum of reactions 1 and 2:                               glucose          +                      2            ⁢                          xe2x80x83                        ⁢            ferricyanide                                                ⟶          GOD                                                  gluconic            ⁢                          xe2x80x83                        ⁢            acid                    ⁢                      xe2x80x83                    +                      2            ⁢            ferrocyanide                                                reaction          ⁢                      xe2x80x83                    ⁢          3                      ⁢      xe2x80x83  
xe2x80x9cGlucosexe2x80x9d refers specifically to xcex2-D-glucose.
The prior art suffers from a number of disadvantages. Firstly, sample size required is greater than desirable. It would be generally preferable to be able to make measurements on samples of reduced volume since this in turn enables use of less invasive methods to obtain samples.
Secondly, it would be generally desirable to improve the accuracy of measurement and to eliminate or reduce variations due, for example, to cell asymmetry or other factors introduced during mass production of microcells. Likewise, it would be desirable to reduce electrode xe2x80x9cedgexe2x80x9d effects.
Thirdly, since the cells are disposable after use, it is desirable that they be capable of mass production at relatively low cost.
According to one aspect the invention consists in a method of manufacture of an electrochemical cell comprising the steps of:
forming an aperture extending through a sheet of electrically resistive material, said aperture defining a side wall of the cell;
mounting a first thin electrode layer to one side of the sheet and extending over the aperture whereby to define a cell first end wall;
mounting a second thin electrode layer to the other side of the sheet and extending over the aperture whereby to define a cell second end wall in substantial overlying registration with the first electrode; and
providing means for admission of a liquid into the cell defined between the side wall and said end walls.
The first and second electrode layers may be conductors or semi-conductors and may be the same or different. Noble metal electrode layers are preferred.
In preferred embodiments of the invention the aperture is of circular cross-section whereby the side wall is cylindrical and the first and second electrodes cover the aperture.